Enable HTML5 Video Firefox

Enable MSE & H.264
A new preference in Firefox Nightly 34 changes that however, so that support for all requested technologies is provided afterwards on YouTube.
Note: While the article concentrates on Google's video hosting platform, enabling support for MSE & H.264 will benefit users of the browser on other websites as well.
You need to create a new preference to do so:
Type about:config and hit enter.
Confirm you will be careful.
Right-click and select New > Boolean.
Name the preference media.mediasource.ignore_codecs.
Set its value to True.
Update: Linux userrs may change the following preferences as well:
media.mediasource.mp4.enabled to true
media.fragmented-mp4.* to true
media.fragmented-mp4.use-blank-decoder to false
If you go back to YouTube's HTML page, you should see all six technologies listed as supported (in green).